Add multiple-output support to `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Add `outputs'
  keyword parameter; pass it to `derivation'.  Define `%outputs' in the
  prologue.

* tests/derivations.scm ("build-expression->derivation with two
  outputs"): New test.

Sort environment variables alphabetically in `derivation'.

* guix/derivations.scm (derivation)[env-vars-with-empty-outputs]: Sort
  the result alphabetically.

* tests/derivations.scm ("build derivation with 1 source"): Add
  environment variables.

Add support for fixed-output derivations in `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Pass HASH and
  HASH-ALGO to `derivation'.

* tests/derivations.scm ("build-expression->derivation for fixed-output
  derivation"): New test.

Add support for fixed-output derivations.

* guix/derivations.scm (read-derivation)[outputs->alist]: For
  fixed-outputs, convert HASH with `base16-string->bytevector'.
  (write-derivation): Likewise, convert HASH-ALGO to a string and HASH
  to a base16 string.
  (derivation-hash): Expect HASH to be a bytevector, not a string;
  convert HASH with `bytevector->base16-string'.

* tests/derivations.scm ("fixed-output derivation"): New test.

Add `base16-string->bytevector'.

* guix/utils.scm (base16-string->bytevector): New procedure.

* tests/utils.scm ("bytevector->base16-string->bytevector"): New test.

Add `base32-string->bytevector' and `nix-base32-string->bytevector'.

* guix/utils.scm (bytevector-quintet-set!,
  bytevector-quintet-set-right!, base32-string-unfold,
  base32-string-unfold-right, make-base32-string->bytevector,
  base32-string->bytevector, nix-base32-string->bytevector): New
  procedures.

* tests/utils.scm ("base32-string->bytevector",
  "nix-base32-string->bytevector"): New tests.

Add `build-expression->derivation'.

* guix/derivations.scm (%guile-for-build): New parameter.
  (build-expression->derivation): New procedure.

* tests/derivations.scm ("build-expression->derivation without inputs",
  "build-expression->derivation with one input"): New tests.

Allow derivations with input derivations.

* guix/derivations.scm (derivation-path->output-path): New procedure.
  (derivation-hash): Call `memoize'.  In the fixed-output case, convert
  HASH-ALGO to a string.  In the other case, sort inputs in the
  alphabetical order of their hex hash.  For inputs with no sub-drvs,
  add "out" as the sub-drv.

* guix/utils.scm (%nixpkgs-directory): New parameter.
  (nixpkgs-derivation, memoize): New procedures.

* tests/derivations.scm ("build derivation with 1 source"): Remove
  useless shebang.
  (%coreutils): New variable.
  ("build derivation with coreutils"): New test.

Fix the order of environment variables in `derivation'.

* guix/derivations.scm (derivation)[env-vars-with-empty-outputs]:
  Prepend new environment variables (as expected by Nix), and add them
  to E.

Test the `build-derivations' operation.

* guix/derivations.scm (derivation): Return DRV as a second value.
* tests/derivations.scm ("build derivation with 1 source"): New test.

First stab at the `derivation' primitive.

* guix/store.scm (%store-prefix): New parameter.
  (store-path?, derivation-path?): New procedures.

* guix/derivations.scm (write-derivation): Pass SOURCES through
  `object->string'.
  (compressed-hash, store-path, output-path, derivation): New
  procedures.

* tests/derivations.scm (%store): New global variable.
  ("derivation with no inputs"): New test.

Add `bytevector->base16-string'.

* guix/utils.scm (bytevector->base16-string): New procedure.

Fix `bytevector->nix-base32-string'.

* guix/utils.scm (bytevector-quintet-ref-right,
  bytevector-quintet-fold): New procedures.
  (bytevector-quintet-fold-right): Add `quintet-fold' parameter; use it
  instead of `bytevector-quintet-fold'.
  (bytevector->base32-string): Pass BYTEVECTOR-QUINTET-FOLD as the
  first parameter.
  (bytevector->nix-base32-string): Pass BYTEVECTOR-QUINTET-FOLD-RIGHT as
  the first parameter.

* tests/utils.scm ("sha256 & bytevector->nix-base32-string"): New test.

Move `sha256' to (guix utils).

* guix/derivations.scm (sha256): Move to...
* guix/utils.scm (sha256): ... here.

store: Raise error conditions upon protocol errors.

* guix/store.scm (&nix-error, &nix-protocol-error): New SRFI-35
  condition types.
  (process-stderr): Raise an error condition upon protocol errors
  instead of returning to the caller.  This allows the connection to be
  reused for further interactions.

Add (guix utils), with `bytevector->base32-string'.

* guix/utils.scm, tests/utils.scm: New files.

Use libchop for cryptographic hashes and related.

* guix/derivations.scm (sha256): Rewrite using libchop's
  `bytevector-hash'.
  (derivation-hash): Add docstring.

Add unit test for derivation parsing and output.

* guix/derivations.scm: Export `read-derivation' and `write-derivation'.

* srfi/srfi-64.scm, srfi/srfi-64.upstream.scm: New files, from libchop.
* tests/derivations.scm, tests/test.drv: New files.

Split (guix) in (guix store) and (guix derivations).

* guix.scm: Move contents to other files.  Just aggregate these.
* guix/derivations.scm, guix/store.scm: New files.